Test execution time analysis in JUnit - Build an Automation Script

Measure and verify test execution time for a sample method

Preconditions (2)

Step 1: Create a test method for the sample method

Step 2: Record the start time before calling the sample method

Step 3: Call the sample method

Step 4: Record the end time after the method call

Step 5: Calculate the elapsed time by subtracting start time from end time

Step 6: Assert that the elapsed time is less than or equal to 1000 milliseconds

✅ Expected Result: The test passes if the sample method executes within 1000 milliseconds, otherwise it fails

Automation Requirements - JUnit 5

Assertions Needed:

Assert that the execution time is less than or equal to 1000 milliseconds

Best Practices:

Use System.nanoTime() for precise time measurement

Use assertions from org.junit.jupiter.api.Assertions

Keep test methods small and focused

Avoid hardcoded sleep or waits in tests

Use descriptive test method names

Automated Solution

JUnit

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertTrue;

public class ExecutionTimeTest {

    // Sample method to test
    public void sampleMethod() {
        // Simulate some work by sleeping for 500 milliseconds
        try {
            Thread.sleep(500);
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    }

    @Test
    public void testSampleMethodExecutionTime() {
        long startTime = System.nanoTime();
        sampleMethod();
        long endTime = System.nanoTime();

        long durationInMillis = (endTime - startTime) / 1_000_000;

        assertTrue(durationInMillis <= 1000, "Execution time should be less than or equal to 1000 ms but was " + durationInMillis + " ms");
    }
}

This test measures how long sampleMethod() takes to run.

We use System.nanoTime() before and after the method call to get precise timing.

The difference is converted to milliseconds.

We then assert the duration is less than or equal to 1000 ms.

If the method runs longer, the test fails with a clear message.

This helps ensure performance expectations are met.

Common Mistakes - 4 Pitfalls

Using System.currentTimeMillis() instead of System.nanoTime()

Hardcoding Thread.sleep() in tests to simulate delays

Not converting nanoseconds to milliseconds for assertion

Not providing a clear assertion message

Bonus Challenge

Now add data-driven testing to measure execution time for three different sample methods with varying sleep durations (200ms, 500ms, 800ms).

Show Hint

Practice

1. What is the main purpose of measuring test execution time in JUnit?

easy

A. To automatically fix failing tests

B. To check if tests are written with correct syntax

C. To verify the correctness of test assertions

D. To identify slow tests and improve test suite performance

Solution

1. Step 1: Understand test execution time concept

   Measuring execution time helps find tests that take longer than expected.

2. Step 2: Identify the goal of measuring time

   Knowing slow tests allows developers to optimize or fix them, improving overall test speed.

3. Final Answer:

   To identify slow tests and improve test suite performance -> Option D

4. Quick Check:

   Execution time = find slow tests [OK]

Hint: Execution time helps spot slow tests to speed up runs [OK]

Common Mistakes:

• Confusing execution time with test correctness
• Thinking it fixes tests automatically
• Ignoring performance impact of slow tests

2. Which JUnit code snippet correctly measures the execution time of a test method?

easy

A. long start = System.nanoTime(); // test code here
long end = System.nanoTime(); System.out.println(end - start);

B. long start = System.currentTimeMillis(); // test code here
long end = System.currentTimeMillis(); System.out.println(end - start);

C. int start = System.nanoTime(); // test code here
int end = System.nanoTime(); System.out.println(end - start);

D. int start = System.currentTimeMillis(); // test code here
int end = System.currentTimeMillis(); System.out.println(end - start);

Solution

1. Step 1: Check correct data type for time

   System.nanoTime() returns long, so variables must be long, not int.

2. Step 2: Verify correct method usage

   System.nanoTime() is preferred for measuring elapsed time with better precision.

3. Final Answer:

   long start = System.nanoTime(); // test code here long end = System.nanoTime(); System.out.println(end - start); -> Option A

4. Quick Check:

   Use long + System.nanoTime() for timing [OK]

Hint: Use long type with System.nanoTime() for precise timing [OK]

Common Mistakes:

• Using int instead of long for time variables
• Using System.currentTimeMillis() for high precision timing
• Mixing data types causing compile errors

3. Given the following JUnit test snippet, what will be printed?

long start = System.nanoTime();
Thread.sleep(100);
long end = System.nanoTime();
System.out.println((end - start) / 1_000_000 + " ms");

medium

A. Approximately 100 ms

B. Exactly 100 ms

C. Approximately 0 ms

D. Compilation error due to Thread.sleep

Solution

1. Step 1: Understand Thread.sleep effect

   Thread.sleep(100) pauses the thread for about 100 milliseconds.

2. Step 2: Calculate elapsed time in milliseconds

   Elapsed time = (end - start) in nanoseconds, divided by 1,000,000 converts to milliseconds, close to 100 ms.

3. Final Answer:

   Approximately 100 ms -> Option A

4. Quick Check:

   Sleep 100ms prints ~100 ms [OK]

Hint: Sleep pauses ~100ms, nanoTime difference shows ~100 ms [OK]

Common Mistakes:

• Expecting exact 100 ms (sleep is approximate)
• Confusing nanoseconds with milliseconds
• Thinking Thread.sleep causes compile error

4. Identify the error in this JUnit test measuring execution time:

@Test
void testMethod() {
  long start = System.nanoTime();
  // test code
  long end = System.nanoTime();
  System.out.println(end - start);
}

medium

A. Missing exception handling for test code

B. Using System.nanoTime() is incorrect for timing

C. No error; code correctly measures time

D. Printing time inside test causes test failure

Solution

1. Step 1: Review timing code correctness

   Using System.nanoTime() before and after test code correctly measures elapsed time.

2. Step 2: Check for syntax or runtime errors

   No syntax errors or exceptions are present; printing time is allowed and does not fail test.

3. Final Answer:

   No error; code correctly measures time -> Option C

4. Quick Check:

   System.nanoTime() timing code is valid [OK]

Hint: Timing code with nanoTime and print is valid [OK]

Common Mistakes:

• Assuming print causes test failure
• Thinking exception handling is mandatory here
• Confusing nanoTime with incorrect timing method

5. You want to fail a JUnit test if it runs longer than 500 milliseconds. Which approach correctly implements this using execution time measurement?

hard

A. Use System.currentTimeMillis() and assert elapsed time <= 500 nanoseconds without conversion

B. Record start and end time with System.nanoTime(), then assert elapsed time <= 500_000_000 nanoseconds

C. Print execution time and manually check after test runs

D. Use Thread.sleep(500) inside the test to limit execution time

Solution

1. Step 1: Understand time units and assertion

   System.nanoTime() returns nanoseconds; 500 ms = 500,000,000 ns.

2. Step 2: Implement assertion on elapsed time

   Calculate elapsed = end - start; assert elapsed <= 500_000_000 to fail if too slow.

3. Final Answer:

   Record start and end time with System.nanoTime(), then assert elapsed time <= 500_000_000 nanoseconds -> Option B

4. Quick Check:

   Assert elapsed ≤ 500ms in nanoseconds [OK]

Hint: Convert 500ms to nanoseconds and assert elapsed ≤ that [OK]

Common Mistakes:

• Using Thread.sleep to limit test time
• Not converting time units correctly
• Only printing time without assertion